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With the release of Firefox 4 Beta 7, Mozilla returned to near the top spot in browser performance rankings.

According to tests run by Computerworld, the new browser is about three times faster than the current production version of Firefox in rendering JavaScript, and lags behind only Opera among the top five browser makers.

On Wednesday, Mozilla launched Firefox 4 Beta 7, a preview that includes all the features slated to make it into the final, polished version next year.

Beta 7 was the first to include Mozilla's new JavaScript JIT (Just In Time) compiler, dubbed "JagerMonkey," which shot the browser's performance into the No. 2 slot behind the alpha of Opera 11.

Computerworld ran the SunSpider JavaScript benchmark suite in Windows Vista Business three times for each browser, then averaged the scores.

Three of the five major browsers were closely bunched at the top. Opera Software's Opera 11 Alpha, which the Norwegian company released last month, barely edged Firefox 4 Beta 7, which in turn narrowly beat the beta of Google's Chrome 8.

Meanwhile, results for Apple's Safari and Microsoft's beta of Internet Explorer 9 put that pair significantly behind the top three. For example, Firefox 4 Beta 7 was approximately 25% faster than IE9 Beta in the trials.

Internet Explorer 8, Microsoft's primary browser, was also tested, although for all intents and purposes, it isn't in the JavaScript race: Firefox 4 was more than 12 times faster at rendering JavaScript than IE8.

Not surprisingly, Mozilla boasted of the speed improvements courtesy of the JagerMonkey compiler, and its integration with the earlier TraceMonkey optimization of the browser's JavaScript engine.

On a company blog telling users to "fasten your seatbelts," Mozilla posted JavaScript results from tests run not only in SunSpider -- a benchmark developed by the team responsible for WebKit, the browser engine that powers Chrome and Safari -- but also in Mozilla's own Kraken and Google's V8.

In two of the three tests, Firefox 4 Beta 7 proved three times faster than Firefox 3.6.12, the current production build. (Mozilla said V8's results showed Firefox 4 was five times faster than 3.6.12.)

Firefox 4 Beta 7 is about three times faster than the older 3.6.12, and lags behind only an alpha of Opera 11 in JavaScript speeds. Smaller results are better.